No hot water, heating fine.

[kp7722]

Hi all,

I recently moved into a new (old) house a few months ago and moved/ changed one of the radiators so I drained the system. At the same time I decided to change it from a gravity fed hot water system to a fully pumped S plan by moving the pump and motorised valves around (more on this later).

Details of current system are as follows: - open vented - 20 year+ old Glow-worm space saver boiler - two Honeywell 2 port motorised valves (making it an S plan) - 2 speed Grundfos pump (set to 2 at all times) - combination of 22mm and 15mm copper pipe all round. 22mm is used to feed CH but where the T piece is after the pump the HW copper pipe is reduced to 15mm to feed into the HW tank.

I originally just moved the radiator and when testing my handywork I noticed that the HW motorised valve wasn't working (stuck open) so i decided to replace this (I never tested the system fully when I first moved in so I can't be sure of how the previous owners used to run the system - the HW motorised valve was set to manual so they must have known it was knackered). After testing the radiator I looked at the pipe layout and decided that it would be straightforward and more efficient to convert it to a fully pumped S plan from a gravity fed HW system. The main reason for this was that the pump was activating when calling for HW only anyway (which actually pumped water around the CH system as well?!) - and the CH motorised valve was actually placed first so you could only get HW if CH was switched on anyway. It was a very bizarre layout and made no sense when operating the controls. The wiring was for a fully pumped S plan system but the pipe layout was for gravity fed (and even that was a botch job).

Anyway I got around the changing all the radiators to TRVs (except for

2 which are lockshields left fully open at all times) and have now testing the system which is detailed below:

- CH only. Boiler fires, pump activates, CH motorised valve opens, CH works fine. - CH + HW. Boiler fires, pump activates, CH and HW motorised valves open, CH works fine and have HW. - HW only. Pump activates, HW motorised valve opens, boiler fires for about 30secs - 2mins then stops. Pump continues to run and HW motorised valve stays open.

Has anybody got any idea as to what could be causing this? It can't be the thermostat on the HW tank as the pump and motorised valve are still both running/open and it works fine when using CH + HW. There is still a little bit or air in the system but it is working it's way out (by bleeding the radiators). No inhibitor has been used yet until I know I won't have to drain the system again. There can't be a blockage or air trap in the HW circuit as I get HW when both CH + HW are switched on. Everything seems fine until the boiler cuts out.

It's driving me crazy!

Many thanks in advance, Paul.

[<me9]

What activates the pump? from your description it sarts before the MVs have opened. It is normal for the micro switch in the MV to operate the pump and boiler. Is the one in the HW MV faulty, and opening again after the motor has fully opened the valve?

[Roger Mills]

On HW-only, is the boiler still receiving a demand - in other words, is it's switched live input live? If not, you've got a wiring fault.

If it *is* receiving a demand - which seems more likely - it means that there is no flow under these conditions - despite the pump running - with the result that the water inside the boiler gets hot, and it then cuts out on its internal thermostat.

You mentioned that "the CH valve was placed first". Is this still the case, or have you corrected it? If the water has to flow through *both* valves to get to the cylinder, but the CH valve is closed, it will give the symptoms you describe!

The water needs to come from the boiler and through the pump. After the pump there needs to be a tee-piece, with the HW valve in one leg and the CH valve in the other so that they are independent of each other. I would suggest that your problem lies in this area.

[kp7722]

Hi Roger,

"The water needs to come from the boiler and through the pump. After the pump

This is exactly how I've changed it to work now. It didn't use to. The CH valve used to sit in front of the T piece so that water from the boiler could only reach the pump or the HW valve if the CH was switched on.

In HW only, the boiler must be receiving a signal for it's demand - it fires and then switches off again. As the other replier has mentioned could it be a faulty microswitch on the (brand new) HW valve? If so then it explains why it opens up okay but the boiler only stays running with CH switched on. What controls the pump? Is it the same microswitch or does it feed straight from the controller? If it's from the microswitch then this doesn't explain why the pump is still running.

Cheers, Paul.

[kp7722]

I think the pump activates at the same time as the valve opens. I'm trying to determine whether or not it's the microswitch in the valve that operates the pump. If it is then I would say the microswitch is okay. If not, then you're right, the microswitch does sound at fault. What are your thoughts on this? Do the valves normally operate the pump or does this come from the main controller?

Cheers, Paul.

[Roger Mills]

The boiler must *initially* be receiving a demand, or it wouldn't fire at all. What I was really asking was, after it shuts down after a couple of minutes, is it *still* receiving a demand? It's important to establish that, because it will tell us what to look at next.

With regard to the boiler and pump control, I can only tell you how an S-Plan is *supposed* to work - there's no guarantee that yours is wired to work in the conventional way! What is supposed to happen is this: The HW demand from the programmer feeds through the cylinder stat to the motor of the HW valve, causing the valve to open. Likewise, the CH demand from the programmer feeds through the room stat to the motor of the CH valve. Each zone valve has a pair of 'volt-free' (which means they're electrically isolated from the motor circuit) contacts. These are wired in parallel, with one side receiving a permanent live feed and the other side being connected to the boiler and pump. So, if either (or both) valve is open, both the boiler and pump get switched on. If you follow the connections on the S-Plan wiring diagram shown at

you'll see what I mean.

One slight variation on that will occur if your boiler requires pump over-run - i.e. if the boiler needs the pump to keep running for a bit after it stops firing in order to carry away the residual heat. In this case, the output from the microswitches will *just* control the boiler and the boiler, in turn, will control the pump - which means that the boiler will need a permanent live feed as well as switched live so that it can run the pump after its demand signal has been switched off - and the boiler will have dedicated 'pump' connections on its connection panel.

Since you seem to have inherited and/or created a bastardised system, I think you need to go back to basics and check *everything* out thoroughly without *assuming* anything!

[kp7722]

at

'll see what I mean.

Thanks for your reply Roger.

You are right in that I inherited a bastardised system. Other DIY projects in my house implies that the previous owner may have taken one of two other shortcuts (I know that's what I'm also trying to do!) You are also right in that I may have created another bastardised system. It looks very logical to me now and the piping makes sense with how it "flows". Water from boiler goes through pump to T piece - one to CH motorised valve, one to HW motorised valve. I will check the operation of each unit again tonight and check to see what each piece is doing at each setting.

One question though, if the HW pipe from the T piece is immediately reduced from 22mm to 15mm would this cause a build up of pressure and therefore overheat the boiler, causing it to cut out? If so, then this could explain how each piece is working the way it does, and why everything is still running when the boiler cuts out. What I plan to do tonight is turn the HW on by itself but set the CH motorised valve to manual (open) to relieve the pressure. If the boiler cuts out then it must be something else, otherwise it surely comes down to pressure?

That Honeywell schematic is the diagram I followed when re-piping the system but I know I shouldn't assume anything!

Cheers, Paul.

[<me9]

Someone else has described S plan (which is what is usual in your circumstances) rather well. It is straightforward. Y (IIRC)plan, which it was probably before is less so. I would double check against a S plan schematic as a first step.

The boiler should be powered via the two zone valve microswitches connected in paralell (live is often orange in, and the switched out is grey, but this does vary with manufacturer). This /should/ power the pump as well, but there may be a pump overrun as described by the other poster. If the flow is seriously restricted, there is no bypass, and/or the main boiler thermostat is faulty you could possibly get the symptoms described. Mechanically opening the zone valves should close the microswitches and fire up the boiler. Check the connections to the zone valves, the microswitches should be connected in paralell, and whenever /either/ valve is opened there should be live to the boiler. As a first step check this, If it is correct, then the fault is likely not to be the zone valves.

How quickly does the hot water heat up when both circuits are on? Does the return from the cylinder heat up when both circuits are on. If teh former heats up quickly, and the cylinder return gets hot, then it is unlikely to be a flow problem.

[Roger Mills]

I wouldn't have thought so. Unless there is an actual blockage, you should be able to get enough water through a 15mm pipe to keep the boiler happy. The boiler will cycle, of course, if the HW cylinder isn't absorbing its full output, but it should run for far more than 2 minutes to start with if the water in the cylinder is cold.

Well, when the boiler cuts out, get a volt-meter and see whether the boiler's 'switched live' (demand) input is still at 240v. That's the only thing which will indicate whether it's a plumbing problem or an electrical problem. [Are you sure that the water in the cylinder isn't hot enough, so the stat is closing the valve and switching off the boiler? If you turn the cyl stat up, does the boiler start again after a few seconds?]

Other things to think about . . . How do the HW and CH returns get back to the boiler? Where are the vent and fill pipes connected into the flow pipe? Did you move them when you converted from gravity HW to fully pumped - 'cos you might need to?! Is there any evidence of 'pumping over' in HW-only mode?

Yes, but what about the wiring diagram? If you'd checked whether your system complied with *that* you wouldn't be uncertain about things like what was controlling the pump!

[Jim Alexander]

not if they are Honeywells, microswitches only close if valve is electrically driven open.

Jim A